Imaging system and method for increasing the dynamic range of an array of active pixel sensor cells

ABSTRACT

The dynamic range of an imaging system that utilizes an array of active pixel sensor cells is substantially increased by reading each cell in the array multiple times during each integration period. Each time a cell is read, the number of photons collected by the cell is saved and the cell is reset if the cell would normally saturate by the end of the integration period. At the end of the integration period, the number of photons collected by each cell is defined by the sum of the values collected during the integration period.

Divisional of immediately prior application Ser. No. 08/711,983, filed Sep. 10, 1996 now U.S. Pat. No. 5,892,541.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to active pixel sensor cells and, more particularly, to an imaging system and method for increasing the dynamic range of an array of active pixel sensor cells.

2. Description of the Related Art

Charge-coupled devices (CCDs) have been the mainstay of conventional imaging circuits for converting a pixel of light energy into an electrical signal that represents the intensity of the light energy. In general, CCDs utilize a photogate to convert the light energy into an electrical charge, and a series of electrodes to transfer the charge collected at the photogate to an output sense node.

Although CCDs have many strengths, which include a high sensitivity and fill-factor, CCDs also suffer from a number of weaknesses. Most notable among these weaknesses, which include limited readout rates and dynamic range limitations, is the difficulty in integrating CCDs with CMOS-based microprocessors.

To overcome the limitations of CCD-based imaging circuits, more recent imaging circuits use active pixel sensor cells to convert a pixel of light energy into an electrical signal. With active pixel sensor cells, a conventional photodiode is typically combined with a number of active transistors which, in addition to forming an electrical signal, provide amplification, readout control, and reset control.

FIG. 1 shows an example of a conventional CMOS active pixel sensor cell 10. As shown in FIG. 1, cell 10 includes a photodiode 12, a reset transistor 14, whose source is connected to photodiode 12, a buffer transistor 16, whose gate is connected to photodiode 12, and a select transistor 18, whose drain is connected in series to the source of buffer transistor 16.

Operation of active pixel sensor cell 10 is performed in three steps: a reset step, where cell 10 is reset from the previous integration cycle; an image integration step, where the light energy is collected and converted into an electrical signal; and a signal readout step, where the signal is read out.

As shown in FIG. 1, during the reset step, the gate of reset transistor 14 is briefly pulsed with a reset voltage (5 volts) which resets photodiode 12 to an initial integration voltage which is approximately equal to the voltage on the drain of transistor 14 less the threshold voltage of transistor 14.

During integration, light energy, in the form of photons, strikes photodiode 12, thereby creating a number of electron-hole pairs. Photodiode 12 is designed to limit recombination between the newly formed electron-hole pairs. As a result, the photogenerated holes are attracted to the ground terminal of photodiode 12, while the photogenerated electrons are attracted to the positive terminal of photodiode 12 where each additional electron reduces the voltage on photodiode 12.

Thus, at the end of the integration period, the number of photons which were absorbed by photodiode 12 during the image integration period can be determined by subtracting the voltage at the end of the integration period from the voltage at the beginning of the integration period.

Following the image integration period, active pixel sensor cell 10 is read out by turning on select transistor 18. At this point, the reduced voltage on photodiode 12, less the threshold voltage of buffer transistor 16, is present on the source of buffer transistor 16. When select transistor 18 is turned on, the voltage on the source of buffer transistor 16 is then transferred to the source of select transistor 18. The reduced voltage on the source of select transistor 18 is detected by conventional detection circuitry.

One problem with active pixel sensor cell 10, however, is that imaging systems which utilize an array of active pixel sensor cells suffer from a limited dynamic range. Conventionally, the dynamic range is defined by the maximum number of photons that a cell 10 can collect during an integration period without saturating (exceeding the capacity of) the cell 10, and the minimum number of photons that a cell 10 can collect during the integration period that can be detected over the noise floor.

The effect of a limited dynamic range is most pronounced in images that contain both bright-light and low-light sources. In these situations, if the integration period of the array is shorted to the point where none of the bright-light information is lost, i.e., where the number of collected photons will not exceed the capacity of the cell during the integration period, then most, if not all, of the low-light information will be lost (resulting in a black image) because the collected photons will not be distinguishable over the noise level.

On the other hand, if the integration period of the array is increased to capture the low-light information, i.e., where the number of collected photons is detectable over the noise floor, then a significant portion of the bright-light information is lost (resulting in a white image) because the number of collected photons will far exceed the capacity of the cell.

One approach to solving the problem of dynamic range is to utilize a non-integrating active pixel sensor cell with a non-linear load device, such as a MOSFET-diode in weak inversion, to obtain a logarithmic response. This approach, however, has a number of drawbacks.

First, the noise in a non-integrating cell is much higher than the noise in a conventional integrating cell (such as cell 10 of FIG. 1). In a conventional integrating cell, the effect of random noise events is averaged over the integration period, while the effect of random noise events in a non-integrating cell can produce substantial distortions. Second, the exact non-linear transfer function of this type of device must be carefully calibrated to avoid variations from cell to cell and due to temperature changes.

Another approach to solving the problem of dynamic range, which is used with CCD systems, is to integrate twice: once with a short exposure and once with a long exposure. For the short exposure, the bright-light information is saved while the low-light information is discarded. Similarly, for the long exposure, the low-light information is saved while the bright-light information is discarded.

The information from the two exposures is then combined to form a composite image. The drawback with this approach, however, is that the resulting image is formed by combining image data from two different periods of time.

Thus, to successfully capture both bright-light and low-light sources in the same image, there is a need for an imaging array of active pixel sensor cells with a substantially increased dynamic range.

SUMMARY OF THE INVENTION

Conventionally, the dynamic range of an imaging system that utilizes an array of active pixel sensor cells is defined by the maximum number of photons that a cell can collect during an integration period without saturating (exceeding the capacity of) the cell, and the minimum number of photons that a cell can collect during the integration period that can be detected over the noise floor. The present invention, however, significantly extends the dynamic range of imaging systems by reading each cell in the array multiple times during each integration period.

An imaging system in accordance with the present invention includes an array of active pixel sensor cells arranged in rows and columns, a plurality of detection circuits that correspond to the number of columns in the array, and an array controller and memory unit.

The imaging system also includes a series of row select lines, a series of cell output lines, and a series of detection output lines. The row select lines are output from the array controller and memory unit to the cells of the array so that each row select line is connected to each of the cells in a row of active pixel sensor cells.

The series of cell output lines are output from the active pixel sensor cells to the detection circuits so that each cell output line is connected to each of the cells in a column of cells and to a detection circuit that corresponds with the column of cells. The series of detection output lines are output from the detection circuits to the array controller and memory unit.

In accordance with the present invention, the imaging system further includes a series of row reset lines which are output from the array controller and memory unit to the cells of the array so that each row reset line is connected to each of the active pixel sensor cells in a row of cells, and a series of column reset lines which are output from the array controller and memory unit to the cells so that each column reset line is connected to each of the active pixel sensor cells in a column of cells.

The series of row and column reset lines allow each cell in the array to be individually reset so that cells which are exposed to light sources which would normally saturate a cell can be read and reset multiple times during each integration period.

Operation of the imaging system of the present invention begins by first resetting each of the active pixel sensor cells in the array. Following this, photons are collected in each of the cells of the array during an integration period that begins after the cells have been initially reset.

During the integration period, each cell is read n times to determine how many photons have been collected in the cell since the cell was last reset. During each of the first n-1 times that each cell is read, the number of photons collected by the cell is compared to a predetermined number.

If the number of photons collected by a cell is equal to or greater than the predetermined number, the number of collected photons is stored and the cell is reset. If the number of photons collected by a cell is less than the predetermined number, the number is disregarded.

When a cell is read for the nth time, the number of photons collected by the cell is combined with any stored number of photons previously collected by the cell to obtain a total number of photons collected by the cell during the integration period. The total number of photons collected by each cell in the array is then output.

Thus, the cells in the array which have been reset n-1 times since the initial reset will collect photons over each of n collection periods of the integration period, while the cells which have never been reset since the initial reset will collect photons over the entire integration period.

A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a conventional active pixel sensor cell 10.

FIG. 2 is a schematic drawing illustrating an imaging system 100 in accordance with the present invention.

FIG. 3 is a schematic drawing illustrating an active pixel sensor cells 110 in accordance with the present invention.

FIGS. 4A-4E are timing diagrams illustrating the operation of imaging system 100 with respect to the cells 110 in the first row of the array in accordance with the present invention.

FIG. 5 is a pictorial representation illustrating the operation of system 100 in a video context with respect to cell C1 of FIG. 2, which is exposed to bright light, cell C2 of FIG. 2, which is exposed to intermediate light, and cell C3 of FIG. 2, which is exposed to low light.

FIGS. 6A-6E are timing diagrams illustrating an alternate operation of imaging system 100 with respect to the cells 110 in the first row of the array in accordance with the present invention.

FIG. 7 is a plan view illustrating a first level of a layout of an active pixel sensor cell 200 that implements the schematic representation of cell 110 in FIG. 3 in accordance with the present invention.

FIG. 8 is a cross-sectional view taken along line 7A--7A of FIG. 7.

FIG. 9 is a plan view illustrating a second level of a layout of active pixel sensor cell 200 in accordance with the present invention.

FIG. 10 is a cross-sectional view taken along line 9A--9A of FIG. 9.

DETAILED DESCRIPTION

FIG. 2 shows a schematic drawing that illustrates an imaging system 100 in accordance with the present invention. As described in greater detail below, the present invention significantly increases the dynamic range of imaging systems by sampling the image cells multiple times during each integration cycle.

As shown in FIG. 2, imaging system 100 includes an array of active pixel sensor cells 110 arranged in rows and columns, a series of detection circuits DC1-DCm that correspond to the number of columns, and an array controller and memory unit 112.

As further shown in FIG. 2, imaging system 100 also includes a series of row select lines RS1-RSn, a series of cell output lines CO1-COm, and a series of detection output lines DO1-DOm. The row select lines RS1-RSn, which correspond to the number of rows of cells, are output from unit 112 to the cells 110 so that each row select line RS is connected to each of the cells 110 in a row of active pixel sensor cells.

The series of cell output lines CO1-COm, which correspond to the number of columns of cells, are output from the active pixel sensor cells 110 to the detection circuits DC1-DCm so that each cell output line CO is connected to each of the cells 110 in a column of cells and to a detection circuit DC1-DCm that corresponds with the column of cells 110. The series of detection output lines DO1-DOm, in turn, are output from the detection circuits DC1-DCm to array controller and memory unit 112.

In accordance with the present invention, imaging system 100 further includes a series of row reset lines RR1-RRn which are output from unit 112 to the cells 110 so that each row reset line RR is connected to each of the active pixel sensor cells 110 in a row of cells, and a series of column reset lines CR1-CRm which are output from unit 112 to the cells 110 so that each column reset line CR is connected to each of the active pixel sensor cells 110 in a column of cells.

The series of row and column reset lines RR1-RRn and CR1-CRm allow each cell 110 in the array to be individually reset so that cells which are exposed to light sources which would normally saturate a cell can be read and reset multiple times during each integration period. FIG. 3 shows a schematic drawing that illustrates an active pixel sensor cell 110 in accordance with the present invention.

As shown in FIG. 3, cell 110 includes a photodiode 122 connected to a first intermediate node N_(IM1), and a split-gate reset transistor 124 that has a drain connected to a power supply node N_(PS), a source connected to node N_(IM1), a first gate SW1 connected to a column reset line CR, and a second gate SW2 connected to a row reset line RR. The advantage of using a split-gate reset transistor is that each cell 110 in the array can be individually reset without resetting other cells 110 in the same row or column.

Cell 110 further includes a buffer transistor 126 and a row-select transistor 128. Buffer transistor 126 has a drain connected to node N_(PS), a source connected to a second intermediate node N_(IM2), and a gate connected to node N_(IM1), while row-select transistor 128 has a drain connected to node N_(IM2), a source connected to a column output lines CO, and a gate connected to a row-select line RS.

FIGS. 4A-4E show timing diagrams that illustrate the operation of imaging system 100 with respect to the cells 110 in the first row of the array in accordance with the present invention. As shown in FIGS. 2, 3, and 4A-4E, operation of an integration cycle begins with unit 112 first resetting the cells 110 in the first row of the array by applying a row reset voltage V_(RR1) to row reset line RR1 at time t₁, followed by the substantially simultaneous application of a series of column reset voltages V_(CR1) -V_(CRm) to the column reset lines CR1-CRm, respectively, at time t₂. (To avoid charge loss, the present invention preferably maintains the row reset voltage V_(RR1) on switch SW2 until the column reset voltage V_(CR1) on switch SW1 has been deasserted).

The row reset voltage V_(RR1) applied to row reset line RR1, which is connected to each of the second gates SW2 of the split-gate reset transistors 124 in the first row, causes first portions of the channels of each of the split-gate reset transistors 124 to invert.

Similarly, the series of column reset voltages V_(CR1) -V_(CRm) applied to the column reset lines CR1-CRm, which are connected to each of the first gates SW1 of the split-gate reset transistors 124 in the first row, causes the remainder of the channels of each of the split-gate reset transistors 124 to invert, thereby causing each of the split-gate reset transistors 124 to become conductive. Once conductive, the split-gate reset transistors 124 pull up the voltages on the photodiodes 122 to an initial integration voltage, thereby ending the reset step.

After the voltages on the cells 110 in the first row have been reset, unit 112 repeats the above-described steps for each of the rows in the array until all of the cells 110 in the array have been reset. Thus, imaging system 100 begins an image collection cycle by resetting each of the active pixel sensor cells 110 in the array to an initial integration voltage.

Once each of the cells 110 in the array have been reset to an initial integration voltage, imaging system 100 next determines the value of the initial integration voltage on each of the cells 110 (due to variations in the threshold voltages of the buffer transistors 126).

As shown in FIGS. 2, 3, and 4A-4E, unit 112 determines the initial integration voltage on each of the cells 110 in the first row by applying a row select voltage V_(RS1), to row select line RS1 at a time t₃, which occurs immediately after the falling edge of the column reset voltages V_(CR1) -V_(CRm).

The row select voltage V_(RS1) applied to row select line RS1, which is connected to each of the gates of the row select transistors 128 in the first row, causes the voltages on the second intermediate nodes N_(IM2) of the cells 110, which represent the initial integration voltages of the cycle, to appear on the cell output lines CO1-COm.

The detection circuits DC1-DCm then amplify and digitize the initial integration voltages on the cell output lines CO1-COm. The digitized initial integration voltages are then output on the detection output lines DO1-DOm to unit 112 which stores the digitized initial integration voltage of each cell 110 in the first row.

Once the initial integration voltage on each of the cells 110 in the first row has been determined and stored, unit 112 repeats the above-described steps for each of the rows in the array until the initial integration voltages on all of the cells 110 in the array have been determined and stored.

After the initial integration voltages on all of the cells 110 in the array have been determined and stored, imaging system 100 begins collecting photons in each of the active pixel sensor cells 110 in the array.

As shown in FIGS. 2, 3, and 4A-4E, from time t₃, which represents the effective beginning of the integration cycle, to a time t₄, which represents the effective end of the integration cycle, light energy, in the form of photons, strikes the photodiodes 122 in the cells 110, thereby creating a number of electron-hole pairs. The photodiodes 122 are designed to limit recombination between the newly formed electron-hole pairs.

As a result, the photogenerated holes in each cell 110 are attracted to the ground terminal of the photodiode 122, while the photogenerated electrons are attracted to the positive terminal of the photodiode 122 where each additional electron reduces the voltage on the photodiode 122 and the second intermediate node N_(IM2) of each cell 110.

Next, in accordance with the present invention, rather than determining the voltages on the photodiodes 122 of the cells 110 (via the second intermediate nodes N_(IM2)) at the end of the integration cycle, as is conventionally the case, the present invention determines the voltages on each photodiode 122 multiple times during each integration cycle.

Thus, in the present invention, each cell 110 is read n times during each integration cycle to determine how many photons have been collected in the cell since the cell was last reset. As a result, n photon values are calculated for each cell 110 in the array where each photon value represents how many photons have been collected in a cell 110 since the cell 110 was last reset.

For digital still cameras, the maximum integration period (image collection period) is defined by the f-stop of the camera. For example, a shutter speed of 1/50 of a second means that the maximum integration period is defined to be approximately 20 mS. On the other hand, the maximum integration period for video cameras is defined by the scan rates of the camera to be approximately 30 mS. The maximum integration period, in turn, defines the lowest light level which can be captured by the camera.

The number of collection periods within each integration period can be set to be any number, but is preferably set to be the minimum number of periods that can be used to prevent a cell exposed to the brightest light from becoming saturated. The advantage of preventing a cell from becoming saturated is that no image information is lost. The advantage of using the minimum number of integration periods is that less noise is accumulated.

FIG. 5 shows a pictorial representation that illustrates the operation of system 100 in a video context with respect to cell C1 of FIG. 2, which is exposed to bright light, cell C2 of FIG. 2, which is exposed to intermediate light, and cell C3 of FIG. 2, which is exposed to low light.

As shown in FIGS. 4A-4E and 5, each integration cycle, which is conventionally set to be approximately 30 mS long, is subdivided into three collection periods of 10 mS. Thus, at the end of the first collection period at time t₅, which is approximately 10 mS after the cells 110 have been reset, unit 112 again applies the row-select voltage R_(RS1) to row-select line RS1 which, in turn, causes the voltages on each of the second intermediate nodes N_(IM2) in the first row, which represent the first integration voltages of the cycle, to appear on the cell output lines CO1-COm.

The detection circuits DC1-DCm then amplify and digitize the first integration voltages on the cell output lines CO1-COm. The digitized first integration voltages for each of the cells 110 in the first row are then output on the detection output lines DO1-DOm to unit 112.

Next, unit 112 subtracts the digitized first integration voltage of each cell 110 in the first row from its digitized initial integration voltage to obtain a first collected photon value for each cell 110 in the first row. The first collected photon value, in turn, represents the number of photons that have been collected by a cell 110 during the first collection period.

Following this, unit 112 compares each first collected photon value to a predetermined value. Although any value can be used as the predetermined value, the value preferably used is one which represents approximately one-half of the maximum number of photons (the pixel fill limit) that can be collected by a cell 110.

If the first collected photon value of a cell 110 is equal to or greater than the predetermined value, then the first collected photon value is stored in unit 112. On the other hand, if the first collected photon value of a cell 110 is less than the predetermined value, then the first collected photon value is disregarded by unit 112.

Thus, as shown in FIG. 5, since cell C1 is exposed to bright light, the first collected photon value of cell C1 exceeds the predetermined value and is, therefore, stored in unit 112. On the other hand, since cells C2 and C3 are exposed to intermediate and low light, respectively, the first collected photon values of these cells are less than the predetermined value and are, therefore, disregarded.

Once the cells 110 having first collected photon values that are equal to or greater than the predetermined value have been determined, unit 112 resets these cells 110. Thus, since cell C1 is the only cell 110 in the first row with a first collected photon value equal to or greater than the predetermined value, unit 112 resets only cell C1 by first applying the row reset voltage V_(RR1) to row reset line RR1 at time t₆, and then applying the column reset voltage V_(CR1) to column reset line CR1 at time t₇.

None of the remaining cells 110 in the first row are reset at this time because both the row reset voltage V_(RR) and the column reset voltage V_(CR) must be present at the same time to reset a cell. Thus, since the column reset voltage V_(CR1) is the only column voltage applied, only the cell in the first row and the first column is reset.

Once the first collected photon values for all the cells 110 in the first row have been determined, and the respective cells have been reset, unit 112 repeats the above-described steps for each of the rows in the array until the first collected photon values have been determined, and the respective cells 110 have been reset for all of the cells 110 in the array.

Following this, photon integration continues until the end of the second collection period at time t₈. At this point, cell C1 has been collecting photons for approximately 10 mS (since it was last reset), while cells C2 and C3 having been collecting photons for approximately 20 mS.

Thus, at time t₈, unit 112 again applies the row-select voltage V_(RS1) to row-select line RS1 which, in turn, causes the voltages on the second intermediate nodes N_(IM2), which represent the second integration voltages of the cycle, to appear on the cell output lines CO1-COm.

The detection circuits DC1-DCm then amplify and digitize the second integration voltages on the cell output lines CO1-COm. The digitized second integration voltages for each of the cells 110 in the first row are then output on the detection output lines DO1-DOm to unit 112.

Next, unit 112 again subtracts the digitized second integration voltage of each cell 110 in the first row from its digitized initial integration voltage to obtain a second collected photon value for each of the cells 110 in the first row. Following this, unit 112 compares each second collected photon value to the predetermined value.

As above, if the second collected photon value of a cell 110 is equal to or greater than the predetermined value, then the second collected photon value is stored in unit 112. On the other hand, if the second collected photon value of a cell 110 is less than the predetermined value, then the second collected photon value is disregarded by unit 112.

Thus, as shown in FIG. 5, since cell C1 is exposed to bright light, the second collected photon value of cell C1 again exceeds the predetermined value and is, therefore, stored in unit 112. In addition, although the first collected photon value of cell C2 was less than the predetermined value, the second collected photon value of cell C2, which is exposed to intermediate light, now also exceeds the predetermined value and is, therefore, stored in unit 112. On the other hand, the second collected photon value of cell C3, which is exposed low light, continues to be less than the predetermined value and is, therefore, disregarded.

In the preferred embodiment, unit 112 stores the first and second collected photon values of cell C1 as a total value, although the first and second collected photon values of cell C1 may alternately be stored in separate locations.

Once the cells 110 in the first row having second collected photon values that are equal to or greater than the predetermined value have been stored, unit 112 resets these cells. Thus, since cells C1 and C2 are the only cells in the first row with a second collected photon value equal to or greater than the predetermined value, unit 112 resets cells C1 and C2 by first applying the row reset voltage V_(RR1) to row reset line RR1 at time t₉, and then applying the column reset voltages V_(CR1) and V_(CR2) to column reset lines CR1 and CR2, respectively, at time t₁₀. None of the remaining cells 110 in the first row are reset at this time because both the row reset voltage V_(RR) and the column reset voltage V_(CR) must be present at the same time to reset a cell.

Once the second collected photon values of the cells 110 in the first row have been determined, and the respective cells 110 have been reset, unit 112 repeats the above-described steps for each of the rows in the array until the second collected photon values for all of the cells 110 in the array have been determined, and the respective cells have been reset.

Following this, photon integration continues until the end of the third integration period at time t₄ which corresponds with the end of the integration cycle. At this point, cells C1 and C2 have been collecting photons for approximately 10 mS, while cell C3 has been collecting photons for approximately 30 mS.

At the end of the third collection period (time t₄), unit 112 again applies the row-select voltage V_(RS1) to row-select line RS1 which, in turn, causes the voltages on the second intermediate nodes N_(IM2), which represent the third integration voltages of the cycle, to appear on the cell output lines CO1-COm.

The detection circuits DC1-DCm then amplify and digitize the third integration voltages on the respective cell output lines CO1-COm. The digitized third integration voltages for each of the cells 110 in the first row are then output on the detection output lines DO1-DOm to unit 112.

Next, unit 112 subtracts the digitized third integration voltage of each cell 110 in the first row from its digitized initial integration voltage to obtain a third collected photon value for each cell 110 in the first row.

Following this, unit 112 combines the third collected photon value with the corresponding first and second stored collected photon values, where present, of each of the cells 110 in the first row to obtain a total collected photon value for each of the cells 110 in the first row. After this, unit 112 outputs the total collected photon value for each of the cells 110 in the first row.

Thus, as shown in FIG. 5, since cell C1 was exposed to bright light during each of the three collection periods, the third collected photon value is combined with the stored first and second collected photon values for cell C1 to obtain the total collected photon value for cell C1. Since cell C2 was exposed to intermediate light, the third collected photon value is combined with the stored second collected photon value for cell C2 to obtain the total collected photon value for cell C2.

On the other hand, since cell C3 was exposed to low light, no first or second collected photon values were stored for cell C3. As a result, the third collected photon value for cell C3 becomes the total collected photon value for cell C3.

Once the third collected photon values for the cells 110 in the first row have been determined, unit 112 repeats the above-described steps for each of the rows in the array until the third collected photon values for all of the cells 110 in the array have been determined.

After the third collected photon values for the cells 110 in the array have been determined, unit 112 begins the next image collection cycle by resetting each of the cells in the array as described above.

Thus, unit 112 begins again by applying the row reset voltage V_(RR1) to row reset line RR1 at time t₁₁, followed by the application of the series of column reset voltages V_(CR1) -V_(CRm) to the column reset lines CR1-CRm, respectively, at time t₁₂. Once the cells 110 in the first row have been reset, unit 112 repeats these steps for each row in the array.

Following this, unit 112 again determines the initial integration voltage on each of the cells 110 in the first row by applying the row select voltage V_(RS1) to row select line RS1 at a time t₁₃, which occurs immediately after the falling edge of the column reset voltages V_(CR1) -V_(CRm). Once the cells 110 in the first row have been measured, unit 112 repeats these steps for each row in the array.

Thus, an imaging system has been described that samples the cells of the array multiple times during each integration cycle. The advantage of sampling each of the cells 110 in the array multiple times is that the dynamic range of the imaging system can be significantly increased.

As shown in FIG. 5, the total collected photon value for cell C1 far exceeds the pixel fill limit (the maximum capacity) of the cell. If this photon information were not summed up in memory, as is conventionally the case, all of the photon information in excess of the pixel fill limit would be lost.

FIGS. 6A-6E show timing diagrams that illustrate an alternate operation of imaging system 100 with respect to the cells 110 in the first row of the array in accordance with the present invention.

As noted above, once the digitized third integration voltages for each of the cells 110 in the first row have been output, unit 112 subtracts the digitized third integration voltage of each cell 110 in the first row from its digitized initial integration voltage to obtain a third collected photon value for each cell 110 in the first row.

As discussed in METHOD FOR OPERATING AN ACTIVE PIXEL SENSOR CELL THAT REDUCES NOISE IN THE PHOTO INFORMATION EXTRACTED FROM THE CELL, filed on even date herewith by Richard Merrill, which is hereby incorporated by reference, unit 112 can subtract the digitized third integration voltage of each cell 110 in the first row from its digitized initial integration voltage taken at the beginning of the second image collection cycle.

Thus, as shown in FIGS. 6A-6E, rather than subtracting the digitized values of cells C1-C3 obtained at time t₄ from the values obtained at time t₃, unit 112 subtracts the digitized values of cells C1-C3 obtained at time t₄ from the values obtained at time t₁₃.

The advantage of reading a cell 110, such as cell C3, immediately before and after the cell has been reset is that the time between successive reads can be reduced from approximately 30 mS (from the beginning to the end of the integration cycle) to approximately 10 μS (from the end of one integration cycle to the beginning of the next integration cycle). Significant results can also be obtained for cells C1-C2 by reducing the time between successive reads from approximately 10 mS to approximately 10 μS. As a result, variations in the voltage on the second intermediate node N_(IM2) due to the threshold voltage variations of buffer transistor 16 that result from 1/f noise can also be substantially reduced.

In addition, as further shown in FIGS. 6A-6E, further reductions in the threshold voltage variations due to 1/f noise can be made by reading the voltage on the cell at times t₁₄ and t₁₅. Thus, the collected photons on cell C1 can be determined by subtracting the digitized value obtained at time t₅ from the value obtained at time t₁₄. Similarly, the number of collected photons on cells C1 and C2 can be determined by subtracting the digitized value obtained at time t₈ from the value obtained at time t₁₅.

With the method shown in FIGS. 6A-6E, an initial measurement must still be made with the digitized value obtained at time t₃ to first determine if the cell is going to be reset at all. Thus, the collected photons on cell C1 will only be determined by subtracting the digitized value obtained at time t₅ from the value obtained at time t₁₄ after a determination has been made that cell C1 will be reset by subtracting the digitized value obtained at time t₅ from the value obtained at time t₃.

Further, the magnitudes of the row and column reset voltages V_(RR) and V_(CR) are preferably set to be sufficiently greater than the voltage on the power supply nodes N_(PS) of the cells 110 so that the voltages on the first intermediate nodes N_(IM1) of the cells 110 are pulled up to the voltage on the power supply nodes N_(PS).

For example, by placing 3.3 volts on the power supply node N_(PS) of a cell 110, and by using 5-volt row and column reset voltages V_(RR) and V_(CR), the split-gate reset transistor 124 of a cell 110 is forced to operate in the linear region which, in turn, pulls up the voltage on the first intermediate node N_(IM1) of the cell 110 to 3.3 volts.

The advantage of forcing the split-gate reset transistor 124 into the linear region of operation during reset is that the voltage on the first intermediate node N_(IM1) is no longer subject to any variation due to changes in the threshold voltage of reset transistor 124 that result from 1/f noise. Alternately, the row and column reset voltages V_(RR) and V_(CR) can be set to be equal to the voltage on the power supply nodes N_(PS), e.g. +5 V.

The magnitudes of the row-select voltages V_(RS1) -V_(RSn) are also set to be sufficiently greater than the maximum voltage on the second intermediate nodes N_(IM2) of the cells 110, such as five volts, so that the voltages on the cell output lines CO will track the voltages on the second intermediate nodes N_(IM2).

As a result, the row-select transistors 128 of the cells 110 will not contribute a significant amount of noise since transistors 128 will also operate in the linear region which is not subject to V_(T) variation. In addition, no current (other than a transient) will flow through transistors 126 and 128 when the row-select voltages V_(RS) are applied because the loads in detection circuits DC1-DCm are capacitive.

FIG. 7 shows a plan view that illustrates a first level of a layout of an active pixel sensor cell 200 that implements the schematic representation of cell 110 in FIG. 3 in accordance with the present invention. FIG. 8 shows a cross-sectional view taken along line 7A--7A of FIG. 7.

As shown in FIGS. 7 and 8, cell 200 is formed on a substrate 210 that includes a trench isolation region 212 defined by an outer wall portion 220 that encloses a plurality of active regions within substrate 210, and a protruding wall portion 222 that extends inward from the outer wall portion 220. Alternately, isolation region 212 can be formed as field oxide regions by the local oxidation of silicon (LOCOS) or other well-known methods.

As further shown in FIGS. 7 and 8, the protruding wall portion 222 divides the plurality of active regions into an L-shaped first active region 230, and rectangularly-shaped second, third, and fourth active regions 232, 234, and 236. The first active region 230 of substrate 210 is doped with an n-type material to form an n+ photodiode 240, while the second, third, and fourth active regions 232, 234, and 236 are doped with an n-type material to form n+ source and drain regions for the transistors of cell 200.

Specifically, with reference to FIGS. 3 and 7, active region 232 functions as the source of row select transistor 128, active region 234 functions as the source of buffer transistor 126 and the drain of select transistor 128, and active region 236 functions as the drains for reset transistor 124 and buffer transistor 126.

In addition, the second and third active regions 232 and 234 are separated by a select channel region 242, the third and fourth active regions 234 and 236 are separated by a buffer channel region 244, and the first and fourth active regions 230 and 236 are separated by a split-gate reset channel region 246.

As further shown in FIGS. 7 and 8, active pixel sensor cell 200 further includes a first layer of polysilicon (poly-1) 216 and an underlying layer of oxide 214 that are formed over a first portion of the split-gate reset channel region 246 and a portion of the outer and protruding wall sections 220 and 222 of isolation region 212.

FIG. 9 shows a plan view that illustrates a second level of a layout of active pixel sensor cell 200 in accordance with the present invention. FIG. 10 shows a cross-sectional view taken along line 9A--9A of FIG. 9.

As shown in FIGS. 9 and 10, active pixel sensor cell 100 further includes a second layer of polysilicon (poly-2) 218 and an underlying layer of oxide 217 that are formed over a second portion of the split-gate reset channel region 246 and a portion of the outer and protruding wall sections 220 and 222 of isolation region 212.

In addition, poly-2 layer 218 and the underlying layer of oxide 217 are also formed over the buffer channel region 244 and a portion of the outer and protruding wall sections 220 and 222 of isolation region 212, and the select channel region 242 and a portion of the outer and protruding wall sections 220 and 222 of isolation region 212.

It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby. 

What is claimed is:
 1. A method for operating an imaging system having a plurality of active pixel sensor cells, the method comprising the steps of:resetting each of the active pixel sensor cells; collecting photons in each of the active pixel sensor cells during an integration period that begins after the cells have been reset; calculating n collected photon values for each of the active pixel sensor cells for photons collected during the integration period, where n is an integer, each collected photon value indicating how many photons have been collected in a cell since the cell was last reset; comparing the first n-1 collected photon values of each of the active pixel sensor cells to a predetermined value; storing each collected photon value of the first n-1 collected photon values of each cell that is equal to or greater than the predetermined value as a stored value; resetting each active pixel sensor cell having a collected photon value of the first n-1 collected photon values that is equal to or greater than the predetermined value; combining the stored value and the nth collected photon value of each cell having a stored value to obtain a total value for each cell; and outputting the total value of each cell having a total value, and the nth collected photon value of each remaining cell.
 2. The method of claim 1 wherein the calculating step includes the steps of:measuring a reset value from each of the cells after the cells have been reset; measuring n integration values from each of the cells during the integration period so that each cell has n integration values and a corresponding reset value; and subtracting, for each of the cells, each of the n integration values from the corresponding reset value to produce n collected photon values for each cell.
 3. The method of claim 2 wherein the measuring a reset value step includes the steps of:reading a reset voltage; and digitizing the reset voltage.
 4. The method of claim 3 wherein the measuring n integration values step includes the steps of:reading n integration voltages; and digitizing the n integration voltages.
 5. The method of claim 1 wherein the calculating step includes the steps of:measuring a first reset value from each of the cells at the beginning of a first integration period; measuring a second reset value from each of the cells at the beginning of a second integration period; measuring n integration values from each of the cells during the first integration period so that each cell has n integration values, a corresponding first reset value, and a corresponding second reset value; subtracting, for each of the cells, each of the first n-1 integration values from the corresponding first reset value to produce n-1 collected photon values for each cell; and subtracting, for each of the cells, the nth integration value from the corresponding second reset value to produce an nth collected photon value for each cell.
 6. A method for operating an imaging system having a plurality of active pixel sensor cells, the method comprising the steps of:resetting each of the active pixel sensor cells; collecting photons in each of the active pixel sensor cells during an integration period that begins after the cells have been reset; calculating n-1 collected photon values for each of the active pixel sensor cells for photons collected during the integration period, where n is an integer, each collected photon value indicating how many photons have been collected in a cell since the cell was last reset; comparing the first n-1 collected photon values of each of the active pixel sensor cells to a predetermined value; resetting each active pixel sensor cell having a collected photon value of the first n-1 collected photon values that is equal to or greater than the predetermined value; recalculating the collected photon values for each of the active pixel sensor cells having a collected photon value equal to or greater than the predetermined value; storing each recalculated collected photon value as a stored value; calculating an nth collected photon value for each of the active pixel sensor cells for photons collected during the integration period; combining the stored value and the nth collected photon value of each cell having a stored value to obtain a total value for each cell; and outputting the total value of each cell having a total value, and the nth collected photon value of each remaining cell.
 7. The method of claim 6 wherein the calculating n-1 photon values step includes the steps of:measuring a first reset value from each of the cells at the beginning of a first integration period; measuring n-1 integration values from each of the cells during the first integration period so that each cell has n-1 integration values and a corresponding first reset value; and subtracting, for each of the cells, each of the first n-1 integration values from the corresponding first reset value to produce n-1 collected photon values for each cell.
 8. The method of claim 6 wherein the recalculating step includes:measuring a second reset value from each cell having a collected photon value equal to or greater than the predetermined value after the resetting the cell for having a collected photon value equal to or greater than the predetermined value so that each cell with a collected photon value equal to or greater than the predetermined value has an integration value and a corresponding second reset value; and subtracting the integration value from the corresponding second reset value.
 9. A method for operating an imaging system having a plurality of active pixel sensor cells, the method comprising the steps of:resetting the plurality of active pixel sensor cells; collecting photons in each of the cells during an integration period that begins after the cells have been reset; reading a collected photon value from each of the cells n times during the integration period, the collected photon value indicating how many photons have been collected in a cell since the cell was last reset; comparing the collected photon value read from each cell during each of the first n-1 times that a cell is read to a predetermined value; storing in memory the collected photon value of each cell having a collected photon value equal to or greater than the predetermined value as stored information; resetting each cell having a collected photon value equal to or greater than the predetermined value; combining the stored information and the collected photon value of each cell read the nth time to obtain a total value for each cell; and outputting the total value of each cell.
 10. The method of claim 9 wherein the reading step includes the steps of:measuring a reset value from each of the cells after the cells have been reset; measuring an integration value from each of the cells each of the first n-1 times that a cell is read so that each cell has n integration values and a corresponding reset value; and subtracting, for each of the cells, each of the n integration values from the corresponding reset value to produce n collected photon values for each cell.
 11. A method for operating an imaging system having a plurality of active pixel sensor cells, the method comprising the steps of:resetting the plurality of active pixel sensor cells; collecting a number of photons in each of the cells after the cells have been reset; reading a first value from each of the cells after the cells have collected photons for a first period of time, the first value indicating how many photons have been collected during the first period of time; comparing the first value read from each cell to a predetermined value; storing in memory the first value of each cell having a first value equal to or greater than the predetermined value; resetting each cell having a first value equal to or greater than the predetermined value; and reading a second value from each of the cells after the cells have collected photons for a second period of time, the second value indicating how many photons have been collected during the first and second periods of time for those cells having a first value less than the predetermined value, and indicating how many photons have been collected during the second period of time for those cells having a first value equal to or greater than the predetermined value, each cell with a first value equal to or greater than the predetermined value having a first value and a corresponding second value.
 12. The method of claim 11 and further comprising the steps of:comparing the second value read from each cell to the predetermined value; storing in memory the second value of each cell having a second value equal to or greater than the predetermined value; and resetting each cell having a second value equal to or greater than the predetermined value; reading a third value from each of the cells after the cells have collected photons for a third period of time, the third value indicating how many photons have been collected during the first, second, and third periods of time for those cells having a first value less than the predetermined value and a second value less than the predetermined value, indicating how many photons have been collected during the first and second periods of time for those cells having a first value less than the predetermined value and a second value equal to or greater than the predetermined value, and indicating how many photons have been collected during the third period of time for those cells having first and second values equal to or greater than the predetermined value, each cell with first and second values equal to or greater than the predetermined value having a first value, a corresponding second value, and a corresponding third value, each cell with first value less than the predetermined value and a second value equal to or greater than the predetermined value having a second value and a corresponding third value.
 13. The method of claim 12 and further comprising the steps of:combining the first value of each cell stored in memory with the corresponding second and third values to obtain a bright value, combining the second value of each cell having a first value less than the predetermined value and a second value greater than the predetermined value with the corresponding third value to obtain an intermediate value; and outputting the second value of each cell having a first value less than the predetermined value, and the third value of each cell having a first value equal to or greater than the predetermined value.
 14. A method for operating an imaging system having a plurality of active pixel sensor cells, the method comprising the steps of:resetting the plurality of active pixel sensor cells to a first reset value; collecting a number of photons in each of the cells after the cells have been reset; determining a first value from each of the cells after the cells have collected photons for a first period of time, the first value indicating how many photons have been collected during the first period of time; comparing the first value read from each cell to a predetermined value; resetting each cell having a first value equal to or greater than the predetermined value to a second reset value; redetermining the first value from each of the cells having a first value equal to or greater than the predetermined value; storing in memory the redetermined first value of each cell; and reading a second value from each of the cells after the cells have collected photons for a second period of time, the second value indicating how many photons have been collected during the first and second periods of time for those cells having a first value less than the predetermined value, and indicating how many photons have been collected during the second period of time for those cells having a first value equal to or greater than the predetermined value, each cell with a first value equal to or greater than the predetermined value having a first value and a corresponding second value.
 15. The method of claim 14 wherein the determining step includes the steps of:measuring the first reset value from each of the cells; measuring an integration value from each of the cells after the cells have collected photons for a first period of time so that each cell has an integration value and a corresponding first reset value; and subtracting, for each of the cells, the integration value from the corresponding first reset value.
 16. The method of claim 15 wherein the redetermining step includes the steps of:measuring the second reset value from each of the cells having a first value equal to or greater than the predetermined value so that each cell having a first value equal to or greater than the predetermined value has an integration value and a corresponding second reset value; and subtracting the integration value from the corresponding second reset value. 